1. Field of the Invention
This invention relates to a gear type manual transmission and a method of assembling the gear type manual transmission.
2. Description of the Related Art
Typically, gear type manual transmissions which are installed to an engine having a crankshaft directed in a longitudinal direction of an engine compartment are equipped with various transmission shafts, including an input shaft, an output shaft and a counter shaft, incorporated in parallel to one another. Such a gear type manual transmission is further equipped with a reverse idler shaft is disposed in parallel to the input and output shafts and a shift mechanism for changing transmission gears range. The shift mechanism incorporates a plurality of synchronizing mechanisms, which are called a synchronizer, to synchronize speeds of gears that are being shifted together. In more detail, such a synchronizer moves ahead of one of the gear unit that is to be meshed, seizes the other of the gear unit and brings the rotational speed of both units together. Once both gears are rotating at the same speed, they may be meshed.
The synchronizer comprises a synchronizer clutch hub with external splines thereon fixedly mounted on a transmission shaft, a front and rear synchronizer rings on both sides of the clutch hub, a gear ring adjacent to each synchronizer ring, and a synchronizer sleeve mounted for axial slide movement on the clutch hub. A shift fork is linked between a shift rod and the synchronizer sleeve to transmit movement of the shift rod to the synchronizer sleeve in the reverse direction, so as to bring gears of a selected gear unit into mesh with each other, thereby select a desired transmission gear with no grinding or shock.
One type of gear shift mechanism for this type of manual transmission comprises a shift arm fixed to a shift rod to transmit movement of the shift rod directly to a gear shift mechanism. Another type of gear shift mechanism has a double armed, reverse shift fork which transmits movement of a shift rod to with an effect of reducing shifting force or shifting distance necessary for the shift rod. The term "reverse shift fork" used in this specification refers to the fact that it transmits movement of the shift rod to a shift mechanism in the reverse direction. In this type of gear shift mechanism, the double armed reverse shift fork is pivotally supported at middle section of both arms by pivot supports, such as pivot pins, which are press-fitted to both side walls of the transmission case, or otherwise secured to a pair of extra support rods extending in parallel from the front end of the transmission case to the rear end as taught in Japanese Unexamined Patent Publication No.54 - 138949.
Typically, when assembling the gear type manual transmission, after having installed shift mechanisms including shift rods and rocker arms or shift forks to a sub-assembly of various transmission shafts, such as an input shaft, a counter shaft, a reverse idler shaft and an output shaft which are essential for the transmission, with gears of each gear unit meshed with each other, the sub-assembly is put as one whole in a transmission case. In the case where a gear type manual transmission employing reverse shift forks in the gear shift mechanism is assembled in that manner, in particular where the reverse shift fork is pivotally supported by pivot pins fitted to both side walls of the transmission case, these pivot pins must be inserted into pivot holes of the reverse shift forks after having put a sub-assembly with the shift rods and the reverse shift forks installed thereto in the transmission case. The reverse shift forks are not always located in correct positions until they are engaged by the pivot pins, it is troublesome, in particular in an automated assembling process, to insert the pivot pins and instal other necessary parts to the transmission.
In the case where the transmission employs a pair of extra support rods for pivotally supporting the reverse shift forks as described in the above mentioned publication No.54-138949, the extra support rods with pivot pins secured thereto are installed to a sub-assembly of transmission shafts and gears, and the reverse shift forks are completely supported by the pivot pins before putting the sub-assembly in the transmission case. While the utilization of extra support rods makes it easy to install the reverse shift forks and has the advantage of automating the process of assembling transmissions, various constraints are imposed on the shift fork support structure. For example, the extra support rod extending from the front of the transmission case to the back needs more space in the interior of the transmission case cram-full of transmission shafts and gears and their associated parts and is too flexible to ensure precise gear shift action.